The present study manipulated mugshot search task instructions to reveal when witnesses make commitment or familiarity based lineup errors. Additionally we examined the memory and decision making processes underlying these lineup choices using a computational model. In order to examine these processes, an extension of Clark's (2003) WITNESS model was developed - WITNESS-ME (ME for Mugshot Exposure). In support of previous research, we found a robust commitment effect. Commitment is due to strong encoding of the committed foil and the differentiation of that choice to the other lineup members. When participants were required to choose several foils that resembled the perpetrator from the mugbook (rather than searching for a single perpetrator), no differences in correct identification between the mugbook and no-mugbook control were found. We also found evidence for errors to due to conscious inference and source monitoring in all mugbook conditions. Modeling these data supported the hypothesis that witnesses are influenced by the number of plausible choices in the lineup and subsequently may adopt different strategies because of this. Theoretical and practical implications are discussed.